This invention relates to a circuit arrangement having a given predetermined overall transfer function, which is defined as the ratio between an output signal which can be taken from an output and an input signal which can be applied to an input.
The article "CAD Simplifies TCXO Design" by Paul Faerber and Alan Victor in "Oscillator Design Handbook" describes a network which can provide compensation for the resonant frequency of a quartz crystal, which is a non-linear function of the temperature. For this purpose an input voltage V.sub.i is converted into a temperature-dependent voltage V.sub.o via said network, which voltage is applied to a varactor diode as a control voltage. This varactor diode is arranged in series with the quartz crystal and enables the resonant frequency of the entire arrangement comprising the varactor diode and the quartz crystal to be adjusted via the input voltage V.sub.i. For the temperature compensation of the quartz crystal the network comprises a plurality of thermistors to permit compensation for the non-linear temperature-versus-resonant-frequency characteristic function of the quartz crystal. Such a network generally requires certain combinations of parts with positive and negative reactive temperature coefficients. In an example given in said article three thermistors with negative temperature coefficients are required in a special combination with further resistors. From said article it is further apparent that such a network can only partly compensate for the temperature-versus-resonant-frequency characteristic function of the quartz crystal.
However, the prior-art circuit arrangements have the drawback that they provide only very inadequate compensation with a highly complex circuit. This high complexity is caused by the fact that the various thermistors to be used cannot be integrated and that adjustment of the resulting network is difficult and complicated. This prohibits a low-cost production of compact temperature-compensated oscillators.
Another temperature-compensation possibility for a quartz crystal is to apply a voltage generated by a digital-to-analog converter to the varactor diode (variable capacitance diode), the input signals for the converter being supplied by a memory. This memory can store a given data sequence for each temperature in order to compensate for the temperature dependence of the quartz crystal by means of the digital-to-analog converter. For this purpose, the memory should be read out in dependence on the temperature of the quartz crystal. Although such a solution in principle enables integration on a semiconductor body and can therefore result in a relatively compact construction, it requires an intricate circuit, particularly if a subsequent adjustment of the temperature dependence is needed.